As the exploration for oil and gas moves further and further offshore, continued innovation is necessary to provide economic solutions to the technical challenges posed by the increased water depths. The method and apparatus of the present invention for stabilizing a floating offshore platform is such a responsive innovation.
The practical depth limit for a fixed bottom-mounted platform is about 1500 feet. Fixed platforms may encounter economic limits, depending on the size of the reservoir, prices of steel and oil, etc., at considerably shallower depths. As a result, the industry is generally moving from fixed platforms to column-stabilized floating platforms such as semi-submersible vessels and tension leg platforms for deep water applications.
A number of significant innovations have been made in conjunction with the creation of a second generation tension leg platform for installation in Green Canyon off the Louisiana coast. These improvements significantly enhance the cost effectiveness of TLPs and include reducing the size and weight of the TLP itself; utilizing a one-piece, thin walled buoyant tendon, described and claimed in commonly assigned U.S. patent application Ser. No. 07/105,941, now U.S. Pat. No. 4,784,529; including a side-entry bottom-installed tendon receptacle to facilitate installation, as described and claimed in commonly assigned U.S. patent application Ser. No. 07/105,942, now U.S. Pat. No. 4,784,529; and providing external mooring porches on the TLP columns, as described and claimed in commonly assigned U.S. patent application Ser. No. 07/105,943, now U.S. Pat. No. 4,784,529.
While these innovations will make TLPs attractive for deeper water usage, they are apt to have practical limits of usage on the order of 4000 feet water depth. Even at depths of 3000 feet, the one-piece buoyant tendon begins to experience limitations. For a conventional steel tendon, the diameter to wall thickness ratio (D/t) for the tendon needs to exceed 29 in order to maintain buoyancy. Yet, the need to increase wall thickness to reinforce the structural integrity of the tendon against possible collapse under hydrostatic pressure at 3000 feet below the ocean's surface, makes it difficult to maintain the necessary D/t ratio.
Without neutral buoyancy, the one-piece tendon design loses some of its attractiveness for comparative ease of transportation and installation. The increased tendon diameters needed for approaching the neutral buoyancy state (2" wall thickness results in 60" diameter) would experience increased drag forces from waves and currents. These fluid drag forces coupled with the tendons' weight produce catenary deflections of the tendons which decrease the angles of departure of the tendons from their connection points to the hull of the TLP. This reduces the effective stiffness of the tension legs, negatively impacting the mooring system's ability to retard surge and sway motions.
The heave flexibility of a tendon is equal to its length divided by the product of its modulus of elasticity and its effective cross-sectional area. As increasing water depth necessitates an increase in the lengths of the tendons, for a particular material, the effective cross-sectional area must increase to prevent an increase in flexibility. However, as already discussed, such an increase in area (or wall thickness) negatively impacts the tendon's weight. Failure to reduce the flexibility, however, results in increased heave, pitch, roll, surge and sway of the platform. Of particular concern is the possibility of increasing a second order motion period into a range subject to harmonic resonance for common wave intervals (typically 3 to 26 seconds). Such a possibility could produce a cataclysmic failure of the tendons and endanger the floating platform and its crew. Hence, resonance must be avoided in spite of the resulting weight penalty and its associated problems.
Reduced buoyancy and increased flexibility each contribute to a need for a larger pretension within the tendons. This, in turn, requires additional reinforcement (i.e., additional weight) at the connection points for the tendons on the TLP columns, to accommodate the additional loading. Further, the costs associated with (a) placing a foundation template to which the tendons attach and (b) the procedure for connecting the tendons themselves, increase appreciably with the depth of the water.
The present invention overcomes the above-mentioned difficulties for all column-stabilized platforms (semi-submersibles and TLP's) by eliminating the need for anchoring the platform to the seafloor in order to control first order motions. An added mass stabilizer is suspended a significant distance (e.g., 500 feet) beneath the column-stabilized floating platform by a plurality of tendons. A stabilizer system can be designed to accommodate a particular floating platform to adequately control first order heave, pitch, roll, surge and sway motions by providing an added mass stabilizer of sufficient submerged weight to maintain the suspending tendons in tension for all load conditions and sufficient added mass (submerged weight plus weight of the volume of water subject to movement when the stabilizer mass moves) to produce the desired behavior, i.e., to "convince" the extrinsic forces that this platform is an anchored platform because it behaves like one. Second and higher order platform motions, which contribute to platform and stabilizer drift, can be resisted by a secondary system such as a spring-buoy mooring arrangement or a dynamic positioning system. When necessary, either the platform or the stabilizer or both could be supplemented with additional mooring.
Various other characteristics, advantages, and features of the present invention will become apparent after reading the following detailed description of the preferred embodiments.